Typical transmission and transaxle oil coolers employ tubular heat exchangers mounted in the outlet tank of the vehicle radiator. These heat exchangers include a cylindrical outer tube, an inner tube and a turbulizer placed in an annular passageway between the inner and outer tubes. Oil is admitted to the annular passageway via an inlet port located at one end of the tube whereupon it passes through the turbulizer and is cooled and exits via an outlet port located near the other end of the tube.
Conventional turbulizers (also referred to as turbulators) which have been used in tubular heat exchangers typically consist of sinusoidal convolutions or rectangular corrugations extending in rows axially along the length of the tubular heat exchanger. Adjacent rows in the flow or axial direction are displaced from one another by half a convolution thereby creating transverse rows of transversely aligned parallel slits or apertures. The function of this geometry is to create artificial turbulence since as the hot oil flows through the heat exchanger and impinges against the leading edge of the corrugations, the resulting excessive form drag splits the oil flow sideways as it advances to the next row of corrugations. This artificial turbulence is on the one hand desirable in that it results in enhanced heat transfer characteristics but is deleterious on the other hand in that it produces a significant contribution to the pressure drop along the axial length of the heat exchanger.
Current design trends in the automotive industry are towards more compact and aerodynamically efficient designs in an effort to increase fuel efficiency and accommodate new accessories such as pollution control devices and the like. This has led to a need to reduce the size of the radiator tank and hence a more compact concentric oil cooler is required. It has been found that down-sizing concentric oil coolers employing conventional turbulizers results in a substantial increase in the pressure drop along the axial length of the cooler. This higher pressure drop can produce deleterious effects on the oil pump thereby reducing the oil circulation rate in the cooling system.
Attempts have been made to minimize the oil pressure drop in the flow direction by eliminating the artificial turbulence. This is achieved by changing the turbulizer orientation so that the corrugations are transversely aligned in circumferential rows with apertures through the corrugations opening in the axial or flow direction thereby forming fluid flow passageways. The resulting structure does not create significant artificial turbulence and therefore cannot strictly be referred to as a turbulizer but is more appropriately termed a fin. The fin is comprised of a plurality of these circumferential rows (also referred to as strips) of corrugations which extend in the axial direction of the tubular heat exchanger. The walls of the passageways are periodically interrupted along the axial or flow direction, and corrugations in adjacent rows or strips have been overlapped by 50% in order to provide a continual restarting of the fluid boundary layers in order to achieve high heat transfer properties. Fins having a geometry wherein adjacent rows or strips of corrugations are offset from each other are typically referred to as offset strip fins (OSF). In this context, offset refers to the fact that adjacent transverse strips are offset from each other by a certain amount such that the corrugations in the adjacent rows overlap to produce the interrupted flow passageways.
Recent theoretical studies (Sparrow, E. M. et al., Transactions of the ASME, February 1977, p.4; and Sparrow, E. M. et al. J. Heat Mass Transfer, Vol. 22, p.1613) suggest that there is considerable potential for achieving increased heat transfer and lower pressure drop using the OSF with the appropriate fin dimensions.